Forecasting the grass pollen season

Even though the grass pollen season won't start until sometime in late October, we're already predicting it will be a heavier than normal grass pollen season this year.

But how on earth do we know that?

As it happens one of our best indicators of the coming season isn't actually here on Earth at all but above us in space.

Satellites.

Satellites regularly take images of our planet at various wavelengths of light. Chlorophyll, the green pigment in plant leaves, absorbs light of particular wavelengths and uses this energy to make food from carbon dioxide and water through a process known as photosynthesis. Measuring how much chlorophyll there is in a plant is a good way of finding out how healthy it is.

Chlorophyll levels can be measured from satellite imagery using the Normalised Difference Vegetation Index or NDVI, a unit-less ratio that ranges between -1.0 and +1.0. Positive NDVI values are associated with a greater density and greeness of plants and NDVI values close to 0 or below are associated with bare soil, urban areas and bodies of water.

Today's picture, taken from the Bureau of Meteorology website, shows Victoria's average NDVI for August.

You'll see that large areas of the state are looking pretty green coming out of winter, especially those grazing and cropping areas to Melbourne's west and north. In short the green areas show that a fair chunk of the state's four million hectares of perennial ryegrass pastures, the source of Melbourne's allergenic grass pollen, is in pretty good health and growing well.

That growth is setting us up for one of the best (or worst, depending on your point-of-view) grass pollen seasons in recent years.

But this outlook isn't locked in. For instance if September rainfall disappoints there will likely be some reduction to the size of the season.

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